In an effort to reduce the amount of pollution emissions from gas-powered turbines, governmental agencies have enacted numerous regulations requiring reductions in the amount of oxides of nitrogen (NOx) and carbon monoxide (CO). Lower combustion emissions can often be attributed to a more efficient combustion process, with specific regard to fuel injector location and mixing effectiveness.
Early combustion systems utilized only diffusion type nozzles, where fuel is mixed with air external to the fuel nozzle by diffusion, proximate the flame zone. Combustors using only diffusion type nozzles produced high emissions because the fuel and air burn stoichiometrically at high temperature to maintain adequate combustor stability and low combustion dynamics.
An enhancement in combustion technology is the utilization of premixing, where the fuel and air mix prior to combustion to form a homogeneous mixture that burns at a lower temperature than a diffusion type flame and produces lower NOx emissions. Premixing can occur either internal to the fuel nozzle or external thereto, as long as it is upstream of the combustion zone. Premixing fuel and air together before combustion allows for the fuel and air to form a more homogeneous mixture, which will burn more completely, resulting in lower emissions.
Although premixing may be effective at high loads and turbine speeds, premixing can have certain drawbacks at low loads and/or low speeds. More particularly, gas turbine engines are required to operate at a variety of power settings and speeds. Where a gas turbine engine is coupled to drive a generator, required output of the engine is often measured according to the amount of load on the generator, or power that must be produced by the generator. A full load condition is the point where maximum output is drawn from the generator and therefore requires a maximum power from the engine to drive the generator. This is the most common operating point for land-based gas turbines used for generating electricity.
However, often electricity demands do not require the full capacity of the generator, and the operator desires for the engine to operate at a lower load setting, such that only the load demanded is being produced, thereby saving fuel and lowering operating costs. Combustion systems of the prior art have been known to become unstable at lower load settings, especially below 50% load, while also producing unacceptable levels of NOx and CO emissions. This condition is especially prevalent during startup prior to achieving a minimum turndown load. This is primarily due to the fact that most combustion systems are staged for most efficient operation at high load settings. The combination of potentially unstable combustion and higher emissions often times prevents engine operators from running engines at lower load settings, forcing the engines to either run at higher settings, thereby burning additional fuel, shutting down, or producing extraordinarily high emissions levels during the startup procedure thereby losing valuable revenue that could be generated from the part-load demand in compliance with emissions regulations.
A problem with shutting down the engine is the additional cycles incurred by the engine hardware. A cycle is commonly defined as the engine passing through the normal operating envelope. That is, by shutting down an engine, the engine hardware accumulates additional cycles. Engine manufacturers typically rate hardware life in terms of operating hours or equivalent operating cycles. Therefore, incurring additional cycles can reduce hardware life and require premature repair or replacement at the engine operator's expense.
What is needed is a system that can provide flame stability and low emissions benefits at a part load condition, as well as at a full load condition, such that an engine can be efficiently operated at lower load conditions, thereby eliminating the wasted fuel when high load operation is not demanded or incurring the additional cycles on the engine hardware when shutting down.